Field of the Invention
Embodiments of the invention relate to a blue phosphorescence compound and an organic light emitting diode using the same, and more particularly, to an organic light emitting diode using a high efficiency blue phosphorescence compound having high triplet energy as a host of a light emitting layer.
Discussion of the Related Art
The importance of flat panel displays is recently increasing with the growth of multimedia. Thus, various types of flat panel displays such as a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and an organic light emitting display have been put to practical use to meet this trend.
The organic light emitting display has advantages in that it can have elements formed even on a flexible substrate formed of, for example, plastic, can be driven at a low voltage of about 10V or less as compared with a plasma display panel or an inorganic light emitting display, and can have comparatively low power consumption and excellent feeling of color. Further, because the organic light emitting display can represent three colors of red, green, and blue, it has attracted the attention of many people as a next generation display representing full colors.
An organic light emitting diode may be formed by sequentially stacking an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode. In the organic light emitting diode, holes injected from the anode and electrons injected from the cathode are recombined with each other to generate excitons. A fluorescence material used as a material of the light emitting layer is concerned in the generation of singlet excitons, and a phosphorescence material used as the material of the light emitting layer is concerned in the generation of triplet excitons. There is a recent trend in which the material of the light emitting is changed from the fluorescence material to the phosphorescence material. This is because only singlet excitons of about 25% among the excitons generated in the light emitting layer formed of the fluorescence material are used to generate light, and most of triplet excitons of about 75% are converted into heat and disappear. On the other hand, most of excitons generated in the light emitting layer formed of the phosphorescence material are converted into light.
In a light emitting process using a phosphorescence element, holes injected from the anode and electrons injected from the cathode meet each other in a host material of the light emitting layer. Further, the holes and the electrons may meet each other in a dopant material of the light emitting layer. However, because a concentration of a host is generally high, the holes and the electrons mainly meet in the host. In this instance, an energy level of singlet excitons generated in the host is transferred to a singlet energy level or a triplet energy level of a dopant, and an energy level of triplet excitons generated in the host is transferred to the triplet energy level of the dopant.
Further, the excitons transferred to the singlet energy level of the dopant are again transferred to the triplet energy level of the dopant through intersystem crossing. Thus, all of the excitons are first transferred to the triplet energy level of the dopant. The excitons thus formed are transferred to a ground state and generate light. When triplet energy levels of the hole transport layer and the electron transport layer positioned in the front or in the rear of the light emitting layer are less than the triplet energy level of the dopant, a reverse energy transition from the dopant or the host to the hole transport layer and the electron transport layer is generated. Hence, efficiency of the organic light emitting diode is sharply reduced. Thus, not only the host material of the light emitting layer but also the triplet energy levels of the hole and electron transport layers greatly affect the phosphorescence element.
For the effective energy transition from the host to the dopant, the triplet energy level of the host has to be greater than the triplet energy level of the dopant. However, because a triplet energy level of carbazole biphenyl (CBP), which is recently widely used as the host, is about 2.6 eV, a reverse energy (heat absorption) transition from the CBP host to a well-known Flrpic phosphorescence dopant is generated. Hence, the efficiency of the organic light emitting diode is reduced. Thus, the development of a new phosphorescence material having a high triplet energy level and excellent thermal stability is urgently needed.